FIX Add tinymce link menuitems per editor instance

[GuySartorelli]

IMPORTANT NOTE

This PR needs https://github.com/silverstripe/silverstripe-admin/pull/1653 to be merged before CI will go green.

The functionality in https://github.com/silverstripe/silverstripe-admin/pull/1653 is required for this PR to work at all - and the code in this PR will not work with earlier versions of silverstripe/admin. Because of this, it should not be merged until that PR has been merged and tagged, and the new constraint has been checked to ensure it is correct.

Description

Updates the asset-admin TinyMCE link plugin to match https://github.com/silverstripe/silverstripe-admin/pull/1653

Manual testing steps

See issue for reproduction steps

Issues

• https://github.com/silverstripe/silverstripe-elemental/issues/1120

Pull request checklist

• [x] The target branch is correct
  • See picking the right version
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  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• [x] The commit messages follow our commit message guidelines
• [x] The PR follows our contribution guidelines
• [x] Code changes follow our coding conventions
• [x] This change is covered with tests (or tests aren't necessary for this change)
• [ ] CI is green